In motors, such as those having a worm and gear, there is a need to prevent the worm and gear from reaching a back driven condition in the event that an external torque is applied at an output end of a gear arrangement or transmission. When such a motor is coupled to a load, a driving torque will act on the motor output drive under certain conditions. This torque can be transmitted from the drive gear to the worm shaft inducing an angular motion on the motor armature (back drive). When this condition is reached, the system ceases to operate as intended; for example, in the case of a window lift motor, this can mean that the window moves downward from an original position without operator intervention under vibration conditions (driving the vehicle). Another scenario causing back drive could be an individual pushing down the glass to gain access to the vehicle.
Currently, back drive prevention is achieved by maintaining a lead angle below a friction angle in the case of worm and gear drives. However, this solution creates an inherent inefficiency in the system that may not be adequate under certain vibration conditions.
There is a need for a bi-directional friction clutch or brake that selectively locks a drive system so that the efficiency of the system can be increased without any major concerns for back drive of the system.